Drill collars are very stiff with a wall thickness of approximately 2" in order that most of the bending takes place in the connections. Consequently, fatigue cracks develop in the drill collar connections. Drill pipe has a thin wall tube and a wall thickness of approximately 3/8" so that most all of the flexing takes place in the tube and not in the connections. Thus, fatigue cracks develop in the tube near the fade out of the upset or protectors. Intermediate weight drill string members are usually referred to as "heavy weight" drill pipe to distinguish between the regular drill pipe and drill collars, and have an approximate 1" wall thickness resulting in a stiffness somewhere between that of drill collars and drill pipe creating characteristics common to both drill pipe and drill collars in that some of the bending takes place in the connections resulting in some fatigue cracks, but not to the degree found in drill collar connections.
In the past, standard heavy weight (thick wall) drill pipe has worked well in vertical or near vertical well bores in non-corrosive environments, but has been less than successful in horizontal wells drilled in high angle and corrosive environments.
Heavy weight drill pipe is used as transition pipe between the heavy drill collars and the relatively light weight drill pipe to prevent shock loads and bending stress from reaching the drill pipe. When heavyweight drill pipe is not used, the drill pipe near the top of the drill collars can suffer severe fatigue damage and failure.
In horizontal drilling, heavy weight drill pipe is run in compression to put weight on the drill bit. When the hole was kicked off more or less gradually, the heavy weight drill pipe was subjected to relatively small bending stresses. Now, however, with the hole being kicked off at 15 to 25 degrees per 100 feet instead of 3 degrees per 100 feet, substantial bending stress is imposed on the heavy weight drill pipe. The pipe, when in compression is also being forced against the side of the hole and subjected to differential pressure sticking.
Additionally, stress corrosion cracking failures in heavy weight drill pipe are increasing due to more corrosive drilling fluids, including the increased use of low-ph, low-solids brine and polymer muds, and the increased presence of hydrogen sulfide and carbon dioxide.
Standard heavy weight drill pipe tubes are made from normalized AISI 1340 carbon steel that has a mixed micro structure with large grains, resulting in a 55,000 psi minimum tensile yield strength and a low impact strength of approximately 15 ft.-lbs. This is a soft material that is not very resistant to fatigue because of the large grain size and low impact strength. Consequently, this micro structure is less resistant to stress corrosion cracking and hydrogen embrittlement.
Standard heavy weight drill pipe tool joints are made from drill collar material which is standard AISI 4145 modified but is then liquid quenched and heat tempered to a high Brinell hardness between 302 and 341. The minimum tensile yield strength on standard heavy weight drill pipe tool joints will run approximately 110,000 psi and its impact strength is approximately 50 ft.-lbs. Although the high hardness of heavy weight drill pipe tool joints is not preferred for hydrogen sulfide service, the tool joints are not as critical as the tubing because the stresses are low in the tool joints when compared to the tubes. However, increased bending stresses in the tube are directly related to the stiffness encountered in standard heavy weight drill pipe tool joints.
Although conventional heavy weight drill pipe addresses reducing fatigue, stress and wear on drill string members used in conventional or deviated well bores by incorporating certain structural features, these features are inadequate for use in high angle and horizontal holes in a corrosive environment. For example, prior art such as U.S. Pat. Nos. 3,773,359 to Chance et al. and 4,811,800 to Hill et al. utilize standard heavy weight drill pipe with upsets or protectors, spiraling in the surface of the upsets and/or hard banding the exterior surface of the protectors which collectively are inadequate for use in a high angle or horizontal well bores that have a corrosive environment. Therefore, there is a specific need for a heavy weight drill pipe that can reduce fatigue in high angle and high angle and/or horizontal well bores that have a corrosive environment.